Deepsea, it's unusual for a mortgage lender to refuse for a timber frame build unless it is of non-standard design or construction.
It is important to check that you will receive a fire rated certificate on completion of the build, this will be for insurance reasons & health and safety.
Do bear in mind that steel framed buildings (while unusual) also require re-building from almost scratch if fire damaged, and concrete block construction would need to be gutted completely, and possibly a certain amount of tearing down depending on the heat damage to the structure.
I've lived in a timber frame home for a number of years now and really have no difficulties whatsoever, from mortgages to insurances, from heat efficiency to utility costs, it's been a success. My best advice to you would be to seek professional advice directly from your prospective mortgage provider, insurers and an engineer. In the construction game there are PLENTY of experts, everyone has an opinion, and loads of rumours. Don't put aside the idea of timber frame just yet!